Roughly 700,000 adolescents and young adults (AYA) are diagnosed with cancer each year in the United States (US), and the incidence of cancer in this cohort has increased steadily over the past 30 years. Nevertheless, improvements in health outcomes for AYA lag far behind the advances that have been made for children and older adults with cancer. Cancer treatments are typically associated with symptoms that substantially impact quality of life including pain, nausea, vomiting, and fatigue. Most patients receive aggressive multimodality therapy with intensive chemotherapy. Recent work has documented significant levels of unmet needs among AYA with cancer, particularly the need for psychosocial support. From a developmental perspective, the demands of cancer and its treatments often directly conflict with the normative developmental needs of AYA, such as living an active, independent life and forming and maintaining new peer relationships. By delivering a psychosocial intervention focused on illness resilience, coping and social support, a mobile-based intervention is an ideal way to complement treatment. Further, smartphone ownership is highest among young people, with the majority using their mobile phones for gathering information about health conditions more so than for any other category of information seeking. The BodiMojo Pocket Coach for AYA with cancer will draw on evidence-based approaches to deliver a psychosocial intervention through a mobile platform. With technical innovations in mobile technologies including gamification, fostering of social ties, and personalized feedback, the proposed intervention will be designed to promote positive coping skills, illness resilience and social support; and to serve as a complement to patient-physician interactions. Importantly, the BodiMojo Pocket Coach for AYA with cancer will build upon an already-established basic technical platform. The aims of the proposed program are (1) to develop a prototype of a psychosocial intervention for AYA with cancer to be delivered through a mobile app; and (2) to pilot test feasibility and acceptance of the program with AYA with cancer and health care providers who work with this population. If successful, the Pocket Coach will represent an innovative and effective mobile health program for AYA with cancer.